1. Field of the Invention
The present invention relates to a driving circuit for a semiconductor power switch. Although suitable for any type of semiconductor power switch, the present invention is especially useful for a gate driving circuit for an insulated gate bipolar transistor.
2. Description of the Related Art
Air conditioners such as centrifugal chillers require a motor to turn an impeller. The motor may be, for example, a 150 to 900 horsepower 3-phase induction motor. Such motors run on alternating current (AC). Generally, the motor is connected directly to a standard AC power line such as a 460 volt power line which causes it to run at a fixed speed.
Because the motor which drives the centrifugal chiller always runs at the same speed, the motor may, at times, run faster than is actually necessary to provide cooling. This in turn causes the efficiency of the compressor to be reduced. Thus, fixed speed centrifugal chillers do not always operate at optimum efficiency. By varying the speed of the compressor in a centrifugal chiller, the efficiency of the centrifugal chiller is substantially increased in comparison to fixed speed operation.
The speed of an induction motor may be varied by changing the frequency of the alternating voltage applied to the motor. Commercial utilities only provide voltage at a fixed frequency, usually 60 hertz. Typically, to vary the speed of induction motors, the 60 hertz AC voltage provided by the utility company is converted to a fixed polarity (DC) voltage. Solid state power switching devices then convert the DC voltage to an AC voltage and apply the AC voltage to the motor. If the switching rate of the switches is varied, the frequency and the magnitude of the AC voltage may be varied, and the speed of the motor will change.
Semiconductor switches provide reliable high speed means to control high currents and high voltages. However, semiconductor switches have limitations on current capacity and voltage withstand capability. Insulated gate bipolar transistors ("IGBT") are high voltage semiconductor switches. IGBTs are, thus, useful in applications requiring fast, high voltage switching.
IGBTs typically include a collector, an emitter, and a gate. An IGBT switches current between the collector and emitter in response to a voltage applied to the gate. Specialized driver circuits apply a driving voltage to the gate of the IGBT. Such circuits typically convert a low voltage, small signal input pulse to a high voltage, low impedance output pulse for driving the gate of the IGBT. To insure fast switching, the driver circuit must accurately deliver the current required by the gate of the IGBT.
IGBTs used as high voltage, high current switches occasionally breakdown which could result in a short circuit across the battery or power supply if another IGBT is gated on. In addition, an inadvertent short circuit might mistakenly be applied across the output terminals of the variable speed drive. Such short circuits will produce a large current which can quickly destroy the semiconductor power switches and other circuit elements if means are not provided to remove the path of current flow.
To protect the power source and associated circuitry, it is desirable to quickly detect a short circuit condition in an IGBT and reduce the voltage applied to the gate of the IGBT to turn off the IGBT. Thus, under short circuit conditions, the gate voltage may be controlled, for example, by the exponential discharge of a capacitor through a resistor. This rate of change is very steep as the IGBT transitions through its active region, and the rapidly changing gate voltage causes the IGBT's collector current to change in a correspondingly rapid manner. Due to stray inductances in the power circuit, a high collector to emitter transient voltage may result.
The inventors of the present invention, however, have recognized that if the rate of change of voltage applied to the gate of the IGBT is reduced, the rate of change of current in the IGBT is also reduced, and the transient voltage developed, across the stray inductance is significantly reduced eliminating the possibility of device destruction due to an over voltage during short circuit conditions.